Fix two favicon issues

[falkoschindler]

This PR fixes the following two issues regarding favicons:

1. The following code should use logo.png as favicon on "/":

   @ui.page('/', favicon='website/static/logo.png')
   def test():
       ui.label('The default favicon is used instead of logo.png!')
   
   ui.run()

   But the route is incorrectly created at "//favicon.ico". By explicitly handling the index path "/" in create_favicon_route(), this bug is easily fixed.

2. The following code should use logo.png as favicon on "/test":

   @ui.page('/test')
   def test():
       ui.label('The favicon is not served.')
   
   ui.run(favicon='website/static/logo.png')

   But there is no favicon route created for "/test", because the page favicon is None. Instead the global favicon should be used. This is fixed by ignoring the page path in get_favicon_url() in this case.